|Description:||Magnified 125X, this photomicrograph revealed the presence of a fertile Ascaris sp. egg that was found in an unstained formalin-preserved stool sample. See PHIL 411 for an example of an unfertilized Ascaris lumbricoides egg.|
The most common human helminthic infection, Ascaris sp.have a worldwide distribution. Their highest prevalence is in tropical and subtropical regions, and areas with inadequate sanitation. Ascariasis occurs in rural areas of the southeastern United States.
Fertilized and unfertilized Ascaris lumbricoides eggs are passed in stool of the infected host. Fertilized eggs are rounded and have a thick shell with an external mammillated layer that is often stained brown by bile. In some cases, the outer layer is absent (known as decorticated eggs). Fertile eggs range from 45 to 75 µm in length. Unfertilized eggs are elongated and larger than fertile eggs (up to 90 µm in length). Their shell is thinner and their mammillated layer is more variable, either with large protuberances or practically none. Unfertile eggs contain mainly a mass of refractile granules. Complete development of the larva requires 18 days under favorable conditions (moist, warm, shaded soil). However, eggs of Ascaris lumbricoides may continue to develop and are infectious even when preserved in formalin.